Rotary actuators are used for many applications in aircraft, particularly for moving movable flaps of the aircraft. These movable flaps are often subject to high aerodynamic forces which feed back into the actuator and can cause damage to the actuator if not properly reacted. Further, it is necessary to be able to hold movable flaps in a given position against these aerodynamic forces.
It is known to provide a no-back device on a screw actuator to prevent feedback forces (i.e. aerodynamic forces on the movable flap that feed into the nut of the screw actuator) from turning the screw shaft undesirably.
For a given handedness of screw thread on the screw shaft, axial loading of the nut in a first direction along the screw shaft will induce clockwise torque in the screw shaft and axial loading in a second direction, opposite the first direction, will induce anticlockwise torque in the screw shaft. This known relationship between the axial direction and induced-torque direction has been used in no-back devices in the past.
Commonly, a shaft will enter the no-back device. This shaft may be the screw shaft itself or may be another shaft connected to the screw shaft such that torque and axial loading on the screw shaft are both transmitted into the shaft, and thus into the no-back device.
The shaft has a radially extending flange. On either side of the flange, in abutment with the flange, is a brake pad. Each brake pad is in abutment (on its side opposite the flange) with a ratchet. Each ratchet is connected (on its side opposite the brake pad) to a thrust bearing in a housing of the no-back device. The housing is fixed to an airframe of the aircraft such that axial and rotational motion of the housing are prevented.
When a feedback force is applied to the nut of the screw actuator, the flange is axially loaded against one of the brake pads and attempts to rotate in a first direction. The brake pad is loaded against a ratchet that is configured to prevent rotation in the first direction (but to allow rotation in the second direction, i.e. the opposite rotational direction). Thus, the brake pad is squeezed between the ratchet and the flange. The brake pad absorbs/reacts against the torque in the first direction (i.e. it acts in the opposite direction), and thus prevents rotation of the screw shaft in response to the feedback force.
Having the ratchet allows the motor that drives the actuator to apply torque to the screw shaft in the second direction. The aforementioned ratchet is able to freely rotate in the second direction and so the motor does not have to work against the friction between the brake pad and the flange when trying to turn the screw shaft. Instead, the motor only has to work against the torque being induced by the feedback forces on the nut. Such no-back devices are known in the art, as described in U.S. Pat. No. 6,109,415 and in U.S. Pat. No. 4,762,205.
In this type of prior art no-back device wear will accumulate in the no-back device, which will eventually require maintenance. However, disassembly and/or removal of the no-back device can often require removal of the actuator as a whole because the no-back device is required to receive axial loading and torque from the screw shaft. The need for disassembly and/or removal of the whole actuator may complicate the necessary maintenance work.
Further, the requirement that the shaft of the no-back device receives both axial loading and torque experienced by the screw shaft limits the design options for where the no-back device may be located.
Such conventional no-back devices for braking actuators against feedback torque have generally been considered satisfactory for their intended purpose. The present disclosure provides a no-back device that may increase design options available to engineers and may allow easier maintenance than prior art devices. Further, with the present arrangement there is no need for the motor to overcome brake friction when the system is driving in an aiding load.